Dr. Luka Kovač is a fictional character portrayed by Croatian-born actor Goran Višnjić. Visnjic's character was added to the cast at the beginning of Season 6 (1999) following the mid-season 5 departure of leading character Doug Ross, played by George Clooney.

Luka is an emergency room attending physician originally from Croatia. He speaks of having a joyful, though financially modest, childhood, with at least one brother, and he keeps in contact with his father, who is an amateur painter and engineer on a Zagreb train line. Luka served in the Croatian Army, and saw combat. He was a family man, with a wife (Danijela), son (Marko), and daughter (Jasna). Jasna was his older child, and from a birthday photo we know she was at least four.

When the Croatian War of Independence (1991–1995) broke out, Luka was at first hesitant to move away, as he wanted to finish his internship. He ended up waiting too long, and it became unsafe to leave. While he was out one morning his apartment building was hit by a mortar shell. His infant son died instantly. His wife and daughter died a few hours later. He carries guilt from the incident because in choosing to stay and perform CPR on his daughter, his wife bled to death, all of which was revealed in the Season 7 episode, The Crossing.

Luka later moved to the United States of America some time after that for a fresh start and later began working as a physician at County General ER but evidence he suffered from both survivor's guilt and PTSD soon emerged, largely in terms of his personal relationships.